Have you searched the issue tracker for a similar issue?
The only other result for modem adapter is Wii network support.

Game Name?
Phantasy Star Online Episode I & II Trial Edition

Game ID?
DPOJ8P

What went wrong?
Cannot get past attached screen, which states "If you want to play PSO Episode 1 & 2 Trial Edition, the modem adapter has to be connected. Restart after switching the power off and connecting the modem adapter."

What did you expect to happen?
It should have booted to the initialization screen where you create a save for the game. I attempted to use the BBA in Dolphin, but research has led me to realize that when this game was initially released, there was no Broadband Adapter in Japan yet. It was released at a later date. The other attachments didn't work either, including "dummy" and the baseboard.

What steps will reproduce the problem?
Simply booting the game. No special settings are needed, although safe textures should be enabled. [If anyone is so inclined, all of the settings from GPOP8P (or any PHANTASY STAR ONLINE EPISODE I&II configuration) can be copied to this game's initialization settings]

What version of Dolphin were you using?
3.5 and -367.

64 or 32 bit Dolphin?
Both will give this error.

What version of Dolphin used to work?
Not applicable.

What Operating System were you using and what are your hardware
specifications?
Windows 8. NVidia 460m.

64 or 32 bit Operating system?
x64.

Any other relevant information or links to logs:
Although modem emulation would be somewhat difficult, I firmly believe that this issue would be fixed with a Modem (Dummy) option for attachment in the SP1 slot.

Thank you very much for accepting this issue. I do not know where to start, so I'll offer my knowledge on the topic.

First off, there is a Wii homebrew application that is amazing: it emulates the modem adapter from the Gamecube on the Wii. This application is called Devolution. The source is here: http://www.tueidj.net/gc_devo_src.zip

Using the source may be a way to at least spoof the hardware on the Dolphin emu, using the SP1 port.

I do not believe that the hardware is region specific. As far as I remember, the hardware was able to be used from Japan on US and EU consoles.